A great variety of detection systems are known in the patent literature and are particularly suited for intrusion detection along a prescribed perimeter. One type of detection system used for this purpose is exemplified in the disclosures of U.S. Pat. Nos. 4,053,877 and 4,091,367 and employs a plurality of leaky co-axial cables which are arranged along the perimeter to be protected and preferably underground. RF signals are transmitted along one of the cables producing a corresponding signal in another one of the cables which is connected to a receiver. The presence of an intruder in the vicinity of the cables results in a change in the received signal, which change is sensed to provide an alarm indication.
The detection system described in U.S. Pat. No. 4,053,877 comprises a single transmitting cable and a pair of receiving cables which are arranged in parallel equally spaced relationship on opposite sides of the transmitting cable. The two receiving cables are connected to a differential transformer which subtracts the two received signals. An alarm signal is provided when the difference between the two signals exceeds a predetermined threshold, at a given time.
The detection system described in U.S. Pat. No. 4,091,367 comprises a pair of leaky coaxial cables which are alternatively coupled to transmitter and receiver circuitry. Apparatus is also provided for locating an intrusion along the length of the cables by sensing the elapsed time until a received signal is received. RF pulses of pulse width 400 nanoseconds and repetition rate 30 KHz are employed. An alarm signal is provided when the difference between the two signals exceeds a predetermined threshold at a given time.
Another type of detection system suitable for installation along a perimeter to be protected comprises an elongate cable which is coupled to a voltage source so as to produce an electric field about its length and a plurality of sensor cables disposed generally parallel to the field generating cable. The presence of an intruder causes the field sensed at any given time by the different sensor cables to change such that the output of a differential detector coupled to the sensor cables produces an alarm indication. A system of this type is illustrated in a publication of the U.S. ARMY, MERADCOM, entitled Report on Sensor Study, July 1, 1977 from pages 368-377. A similar system is shown in U.S. Pat. No. 4,174,518.